It is known that the pulsating pressure of blood delivered from the heart of a person has a completely rectangular waveform. Preferably, therefore, the driving apparatus switches the pressure from positive pressure to negative pressure and vice versa at high speeds in order that the pulsating pressure of blood delivered from a blood-pumping device approach a rectangular waveform as close as possible.
The speed at which the pressure is switched from positive pressure to negative pressure or vice versa increases as the mass of the shuttle gas filling the inside of the blood-pumping device decreases. For this reason, it is desired to use helium gas or other gas having small mass as the shuttle gas. As an example, U.S. Pat. No. 4,175,264 issued Nov. 20, 1979 discloses a driving apparatus that uses helium gas as the shuttle gas.
In a blood-pumping device, the shuttle gas is separated from blood by a thin membrane and so the water vapor contained in the blood dissolves into the shuttle gas through the membrane. This increases the mass of the shuttle gas. Also, the water vapor dissolved in the shuttle gas changes into droplets of water, thus hindering movement of the shuttle gas.
Therefore, it is the common practice to stop the blood-pumping device for a short time on a doctor's or nurse's judgement. Then, water gathering in the blood-pumping device is removed by the doctor or nurse. In order to maintain the performance of the blood-pumping device, the removable of water has been required heretofore. For this purpose, the doctor must constantly monitor the device.